Not included. CSS selectors include: 1. Tag selector, which locates specific HTML elements through the element name of the HTML page; 2. Class selector, which locates specific HTML elements through the value of the class attribute of the HTML element; 3. ID selector, which Locate specific HTML elements through the value of the id attribute of the HTML element; 4. The wildcard selector "*" can refer to all types of tag elements, including custom elements; 5. The attribute selector uses the existing attribute name of the HTML element or attribute value to locate a specific HTML element.

The operating environment of this tutorial: Windows 7 system, CSS3&&HTML5 version, Dell G3 computer.

CSS selectors do not include "hypertext tag selectors", but include class selectors, tag selectors, ID selectors, attribute selectors, pseudo-class selectors, etc.

What is a css selector?

Style is the smallest syntax unit of CSS. Each style contains two parts: selector and Statement (rule), as shown below.

1. Selector (Selector)

The selector consists of the id, class attribute or element name itself of the HTML element and some Special symbols are used to specify which HTML element to define the style for. For example, the selector p means to define the style for all

tags in the page;

2. Declaration

There can be one or countless declarations. These declarations tell the browser how to render the object specified by the selector. All declarations are placed within a pair of curly braces { }, and then the entire declaration is placed immediately after the selector.

The statement must include two parts: attributes and attribute values, and use a semicolon to mark the end of a statement. The semicolon can be omitted for the last statement in a style.

• Attribute: The style name you want to set for the HTML element, consisting of a series of keywords, such as color, border, font, etc., in CSS Provides many attributes, which you can view through the W3C official website;

• Value: consists of a numerical value and a unit or keyword, used to control the display effect of a certain attribute, such as the value of the color attribute It can be red or #F1F1F1 etc.

#What are the css selectors?

We start with an Html structure

<div id="box">
    <div class="one">
        <p class="one_1">
        </p >
        <p class="one_1">
        </p >
    </div>
    <div class="two"></div>
    <div class="two"></div>
    <div class="two"></div>
</div>

Commonly used selectors for css are:

• ##id selector (# box), select the element with the id of box

• class selector (.one), select all elements with the class name one

• tag Selector (div), selects all elements with the label div

• Descendant selector (#box div), selects all div elements with the id inside the box element

• Child selector (.one>one_1), selects all .one_1 elements whose parent element is .one

• Adjacent sibling selector (.one .two ), select all .two elements immediately after .one

• Group selector (div, p), select all elements of div, p

• Pseudo-class selector

:link ：选择未被访问的链接
:visited：选取已被访问的链接
:active：选择活动链接
:hover ：鼠标指针浮动在上面的元素
:focus ：选择具有焦点的
:first-child：父元素的首个子元素

• Pseudo element selector

:first-letter ：用于选取指定选择器的首字母
:first-line ：选取指定选择器的首行
:before : 选择器在被选元素的内容前面插入内容
:after : 选择器在被选元素的内容后面插入内容

• Attribute selector

[attribute] 选择带有attribute属性的元素
[attribute=value] 选择所有使用attribute=value的元素
[attribute~=value] 选择attribute属性包含value的元素
[attribute|=value]：选择attribute属性以value开头的元素

• Hierarchical selector (p~ul), selects each ul element preceded by a p element

• Pseudo-class selector

:first-of-type 表示一组同级元素中其类型的第一个元素
:last-of-type 表示一组同级元素中其类型的最后一个元素
:only-of-type 表示没有同类型兄弟元素的元素
:only-child 表示没有任何兄弟的元素
:nth-child(n) 根据元素在一组同级中的位置匹配元素
:nth-last-of-type(n) 匹配给定类型的元素，基于它们在一组兄弟元素中的位置，从末尾开始计数
:last-child 表示一组兄弟元素中的最后一个元素
:root 设置HTML文档
:empty 指定空的元素
:enabled 选择可用元素
:disabled 选择被禁用元素
:checked 选择选中的元素
:not(selector) 选择与 <selector> 不匹配的所有元素

• Attribute selector

[attribute*=value]：选择attribute属性值包含value的所有元素
[attribute^=value]：选择attribute属性开头为value的所有元素
[attribute$=value]：选择attribute属性结尾为value的所有元素

css basic selector and priority

css basic selector

优先级

相信大家对CSS选择器的优先级都不陌生：

内联 > ID选择器 > 类选择器 > 标签选择器

到具体的计算层面，优先级是由 A 、B、C、D 的值来决定的，其中它们的值计算规则如下：

• 如果存在内联样式，那么 A = 1, 否则 A = 0

• B的值等于 ID选择器出现的次数

• C的值等于 类选择器 和 属性选择器 和 伪类 出现的总次数

• D 的值等于 标签选择器 和 伪元素 出现的总次数

这里举个例子：

#nav-global > ul > li > a.nav-link

套用上面的算法，依次求出 A B C D 的值：

• 因为没有内联样式 ，所以 A = 0

• ID选择器总共出现了1次， B = 1

• 类选择器出现了1次， 属性选择器出现了0次，伪类选择器出现0次，所以 C = (1 + 0 + 0) = 1

• 标签选择器出现了3次， 伪元素出现了0次，所以 D = (3 + 0) = 3

上面算出的A 、 B、C、D 可以简记作：(0, 1, 1, 3)

知道了优先级是如何计算之后，就来看看比较规则：

• 从左往右依次进行比较 ，较大者优先级更高

• 如果相等，则继续往右移动一位进行比较

• 如果4位全部相等，则后面的会覆盖前面的

经过上面的优先级计算规则，我们知道内联样式的优先级最高，如果外部样式需要覆盖内联样式，就需要使用!important

下面我们来看一个实例

nbsp;html>


    <meta>
    <title>这是一个demo</title>
    <style>
        #myid{color:red;}
        .myclass1{color:yellow;}
        h1,p {color:green;}

    </style>

    
        <h1 id="这是一个标题-请查看优先级">这是一个标题，请查看优先级</h1>
        <p>这是一个段落，请查看优先级</p>
    

我们可以看到因为标签

有行内样式，所以它显示为了蓝色；

而标签

虽然定义了三种css样式，但是由于id选择器的优先级最高，所以显示为了红色

（学习视频分享：web前端）

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